With an increase in the number of X-ray detector arrays, X-ray computed tomography apparatuses tend to frequently use volume data as images to be reconstructed. In order to reconstruct such volume data, projection data corresponding to one rotation around an object, i.e., 360°, is required, or (180°+fan angle) projection data is required in the half scan method. On the boundary between volume data, there are regions having no projection data for reconstruction in a field of view. Each region having no projection data on the boundary between volume data is provided with a mask to cover the region.
There is available a technique of combining and displaying a plurality of volume data along a direction perpendicular to slice surfaces to display a range wider than one volume data.
For example, as a technique to be used when the cone angle of an X-ray cone beam in volume scanning is small, there is available a technique of interpolating pixel values in a region having no volume data by performing weighted addition of a plurality of discrete volume data obtained in advance along a direction perpendicular to slice surfaces and creating a slice image from the interpolated pixel values. This technique interpolates a region having no volume data by an approximate method. In some cases, therefore, a good image cannot be obtained.
Another conventional technique is to, for example, cut the above mask regions near the boundary between two volume data and combine the two cut volume data, thereby combining the two volume data. In this case, simply combining the two volume data may lead to a failure to obtain a good image because of the generation of an apparent boundary (step) between CT values at the joint portion due to the difference in image quality between the two volume data.